4: Contamination: The measuring cylinder should be washed in between
filling up the beakers as one solution will contaminate the next.
5: Average: To make the experiment as accurate as possible the experiment
will be repeated 3 times and then an average taken.
6:Temperature: The temperature may affect the reliability of the experiment
for example at extreme temperatures the cells of the potato may die and at
less extreme temperatures the experiment may be speeded up. To keep this
from happening, all the beakers will be kept in the same place and at the
same time of the one-hour experiment.
Apparatus:
* Cork Borer (diameter of 0.5cm),
* Distilled water,
* forceps,
* Measuring cylinder,
* Tile,
* Potato,
* 0.1m, 0.2m, 0.3m,0.4m, 0.5m, 0.6m sugar solutions 200cm3 of each,
* Weighing boat,
* Weighing balance,
* Beakers,
* Blotting paper,
* Marker pen,
* Ruler,
Diagram:
Prediction:
When two solutions (water in potato and sugar solution) are divided by a
semi-permeable membrane the water will move from the area of high
concentration to the area of low concentration, until both sides are equal
(equilibrium).
The tiny holes in the membrane of the potato cells will allow the water
molecules to pass through in and out of the cell, depending on the
concentration gradient of the solution. So in this case, when the water
concentration is lower in the tissue, the water will go inside the potato cells,
and the potato sample will gain in mass. If there is very little difference in the
two water concentrations, there shouldn’t be such a big change in mass. Also
if there is a higher concentration of water in the potato cells, then the water
will go out of the potato and into the liquid solution.
Potato cells will take in water when the surrounding solution has a higher
water concentration than the cell contents. The extreme would be distilled
water and I would expect the mass of the potato to increase the most. In the
highest concentrations of sugar solution I would expect the potato mass to
decrease. Somewhere in between distilled water and 0.6 moles I would
expect the potato mass neither to increase nor to decrease as the cell
contents and the concentration of sugar solution will be the same
concentration i.e. at equilibrium.
Here I have a prediction graph to show what I think a graph showing the
differences between masses will look like.
Method:
1. Bore holes into the potato and then cut them into 2cm long cylinders.
2. Make need 21 of these cylinders.
3. Take 7 beakers.
4. Fill one with 200cm3 of distilled water, label it with a marker pen “distilled
water”.
5. Do the same for 0.1 - 0.6 moles of sugar solution.
6. Take 7 of the potato cylinders and mark them with an X.
7. Take another 7 and colour the base in.
8. Take the last 7 an put a large dot on them.
9. Record the mass of each of the cylinders and record in a table.
10. Place one of each of the differently marked cylinders in each of the
beakers and record which order they went in.
11. An hour later take the first cylinder you put in and take some blotting paper
and remove as much of the surface liquid as possible.
12. Put the cylinder on the scales in the weighing boat and record the
measurement.
13. Return the cylinder to its beaker
14. Do points 11 to 13 for all the other cylinders.
15. Then another hour later do points 11 - 14 again.
Results:
Original mass
(g)
2nd mass
(g)
3rd mass
(g)
Distilled water
1.983
2.27
2.36
Distilled water
1.95
2.18
2.27
Distilled water
1.94
2.11
2.3
average
1.96
2.19
2.31
0.1 mole
1.98
2.23
2.39
0.1 mole
1.95
2.14
2.32
0.1 mole
1.81
2.05
2.1
average
1.91
2.14
2.27
0.2 mole
1.69
1.86
1.86
0.2 mole
1.91
2.08
2.06
0.2 mole
1.91
2.84
2.32
average
1.84
2.01
2.08
0.3 mole
1.86
2.05
1.96
0.3 mole
1.85
2.08
1.97
0.3 mole
1.89
2.09
1.98
average
1.87
2.07
1.97
0.4 mole
1.84
1.73
1.86
0.4 mole
1.85
1.95
1.78
0.4 mole
1.91
1.95
1.74
average
1.87
1.88
1.79
0.5 mole
1.82
1.57
1.74
0.5 mole
1.89
1.69
1.48
0.5 mole
1.84
1.66
1.66
average
1.85
1.64
1.63
0.6 mole
1.76
1.46
1.48
0.6 mole
1.93
1.62
1.6
0.6 mole
1.93
1.61
1.55
average
1.87
1.56
1.54
Analysis
Chart 1
My prediction was correct. The distilled waters potato mass increase showing
that it took on water. Whereas the most concentrated sugar solutions potato
mass decreased.
In between the two extremes the gaining mass decreased as the sugar
solution became more concentrated.
Beyond 0.4 moles the mass decreased below the original mass.
Chart 2
There is a straight line between 0.1 and 0.6. This straight line crosses roughly
in the middle, between 0.3 and 0.4, roughly estimated at 0.35.
At 0.1 you can that is where is maximum turgidity as you would expect the
line to continue up to distilled water but instead it levels out. This means that
the cells were unable to take any more water due to the cell wall.
The graphs and the results show that:
* Osmosis actually took place in the experiment.
* As the moles increased, the percentage of the weight difference
decreased.
There are fairly interesting anomalous results at 0.3 and 0.4. At this level the
3rd mass is not that different from the original mass. 0.3 goes up a little and
0.4 goes down a little.
* all of the original 0.3 readings went up to the 2nd mass.
* then the 2nd mass went down to the 3rd mass but still above the original.
* in two of the 0.4 readings went up for the 2nd
* and then down (even bellow the original) to the 3rd.
I have no explanation for these anomalous results but to help I might have to:
* repeating the experiment
* further work
Evaluation
Looking at the overall experiment I have thought of a number of
improvements to give me more and better accurate results. Also when the
potato was dried to remove surface liquid it was not necessarily done the
same on each potato, a more accurate and uniform way of drying would
improve the accuracy further. Another thing I could have to improve my
resulting was to measure the diameter change of the potato, which would
have helped me to find out the volume before and after of the potatoes, which
would also help me to explain the results obtained.
To make this experiment better, I believe that I could have done one test at a
time, so that I can reduce the time difference, when I have to move the potato
from the beaker to the balance. Between this, I have to dry the potatoes just
enough, and then put it on the balance. When I'm doing this for one set,
writing down the results at the same time, while the other 5 sets are on the
tissue paper, the water outside the potato tissue is going to vary for all.
Therefore, I would be able to concentrate more on one of the sets, instead of
trying to finish all of them as quickly as I can.
I also could have got more people to do the experiment with me, so that I can
organise the tasks, and we would then be able to divide the tasks helping us
to get more accurate results.
Using more types of molar sugar solutions would have helped us obtain better
results, and more accurate results, so that we can make sure the results are
totally correct.
Experimenting with one set for a longer period of time, for each set, would
lead us to better results, because the osmosis action would reach its
maximum capability, and therefore tells us how much water could be
transferred for each solution.
Stirring may help as it would stop a concentration around the sample
Further work could be carried out to include concentrations that increased in
0.05 M rather than 0.1. This would increase the accuracy and improve the
graph. Other investigations could include using different varieties of potato or
different plant tissues e.g. carrot, apple.
I could also extend this experiment by repeated exactly as before. However
this time I could take more results at the mole levels 0.20, 0.21, 0.22,0.23,
0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30. This would produce much more
accurate results.
Other variables in the experiment could be changed for example instead of
changing the mass of the potato the species of the potato could be changed.
For example I could use a new potato and a really old potato.
Also the shape and size could be changed. However this would not affect the
results much. This is because the variable would only change the rate of
osmosis because of a different weight and size.
Temperature could also be changed for example the samples could be
placed in different water baths and brought up to different temperatures to
see if temperature played its part in the osmosis of potatoes. 5 sets of 5
potatoes could be placed in water baths at 10 C, 20 C, 30 C, 40 C, 50 C and
60 C.
I could also as I mentioned earlier, investigate into the results I got for 0.3 and
0.4 moles.
